Burns are judged by the size of the burn in relation to the whole body and by the depth of the burn injury. Different methods exist to calculate the extent or size of a burn injury. The most common method, which provides a quick estimate of burn size, uses the "Rule of Nines," where the body is divided into areas equaling multiples of nine percent of the total body surface area.

The palm of your hand, for example, is equal to about one percent of your body’s surface area. The head and arms are each equal to nine percent of the body surface. The chest and back are each 18 percent (two X nine percent). Each leg is 18 percent (two X nine percent). This totals 11 nines, or 99 percent.